Devices for transmitting torque between two rotatable shafts are known from numerous patent publications. The two shafts may be the output shafts of a road vehicle differential mechanism. The device may accordingly in this case be called a differential brake. There may, however, be other instances when it is desired to counteract a certain rotational speed differential between two shafts, a typical example being the two shafts to the driven front and rear axle, respectively, of a four-wheel-drive vehicle.
There may be different techniques to obtain the counteracting of the rotational speed differential, but several of the known solutions rely on a hydraulic system.
A device of the kind defined is shown in WO 98/26950 and may be defined as an electromechanical device, as the braking means used therein are electrical or electromagnetical, which means that the normally used hydraulic system is dispensed with.
This device comprises                gear means connected to the first shaft member,        a pressure plate, which is connected to the second shaft member and is axially displaceable against the clutch discs for their engagement, and        transmission means between the gear means and the pressure plate for axially displacing the pressure plate against the clutch discs at a certain rotation differential between the gear means and the pressure plate,        the braking means being connected to the gear means for providing a braking force to the transmission means for its axial displacement.        
In this device the gear means is of the planetary gear type. In a comparatively conventional way, it comprises gear wheels cooperating with the first shaft member, planet wheels, a planet holder device, a fixed gear rim, and a gear ring in engagement with the braking means.
Planetary gear devices are comparatively expensive, which may be a drawback in the road vehicle industry, where costs have to be kept down. They also tend to emit noise, which is a further drawback in a road vehicle environment.
The object of the invention is to provide an arrangement without the mentioned drawbacks but maintaining the reliability in construction and function of the planetary gear device.